In Japanese Industrial Standards, the strength of couplings that should be used is defined for connected pressure piping. In particular, concerning the strength of couplings, the strength that the base material of the pressure piping connected to the coupling must have before failure in a tension test is defined. In addition, with respect to a connection by welding, the pressure piping is required to extend a leg length for all over the place in order to keep weld strength that the base material of the pressure piping before failure in the same way. Such welding work requires skill, and therefore, the number of persons able to perform the work is limited.
Different connecting means than welding includes adhesive and bolts. For example, patent literatures 1 and 2 disclose means for combining pipe and pipe by bolts. These technologies are to fix pipes for accommodating electric wires with the fastening force of bolts screwed on peripheral surface walls of the pipes in their radial direction. These pipes are to protect the inner electric wires, and the electric wires in themselves do not either have pressure or leak. Patent literature 3 discloses a technique for connecting poles with the bolts, wherein the poles are fixed with the fastening force of the bolts.